This invention relates to the nuclear reactor art and has particular relationship to nuclear reactors which serve as primary sources for power-supply facilities. A demand which is imposed on such reactors is that they be refueled periodically. Refueling operations carried out in accordance with the teachings of the prior art consume about 3 to 6 weeks.
While this invention is applicable to reactors of other types, it is uniquely applicable to the refueling of reactors of the pressurized water type, P.W.R. In the interest of concreteness this application, in its descriptive text, confines itself to P.W.R.'s assuming the pressure vessel to be vertical.
Such nuclear reactor (U.S. Pat. No. 3,607,629 above) includes a pressure vessel having a body sealed by a head. The body is typically 40 feet long and 15 feet in diameter and includes the fuel core, which typically may include 193 fuel assemblies and the upper and lower internals. The head is typically 15 feet in diameter and in the past has been sealed by about 52 studs.
The reactor includes control rods which are inserted in, or retracted from, the fuel for control purposes by control rod control mechanisms. The mechanisms operate in housings which are sealed pressure tight to the head and extend above the head. There are typically 60 such mechanisms each including drive or drives, typically a piston, a control rod drive shaft connected to the drive and extending from the mechanism housing through the head, each shaft engaging associated control rods.
In refueling, in accordance with the teachings of the prior art, the studs are detensioned and removed from the vessel flange and the head and mechanism housing are lifted and removed exposing the control rod drive shaft and the control rods. The control rod drive shafts are then disconnected from the control rods and removed with the upper internals and the refueling is carried out with the control rods in the core. After refueling the above described process is reversed.
The long shut down of several weeks which this method of refueling demands renders refueling at frequent intervals not practicable and the refueling in accordance with the teachings of the prior art takes place approximately annually.
It has been discovered in arriving at this invention that this annular refueling imposes severe restrictions on the initial or replacement fuel. Typically, the initial enrichment in fissionable material of the fuel must be sufficient to maintain the reactivity of the reactor for at least a year. Typically, this enrichment is of the order of 3.2%. Because of this higher initial enrichment the medium in which the fuel is immersed must have a higher concentration of neutron absorber such as boron. These conditions are imposed not only on the initial fuel but also on each replacement. In addition, the number of fuel assemblies replaced is based on the annual refueling cycle and must be a substantial fraction of the assemblies in the core.
Evaluation of the economic effects of refueling time reveals that there is large economic incentive in reducing materially the refueling time. Not only can the loss, resulting from the reactor being out of operation for long intervals, be reduced, but, in addition, because the refueling can take place at short intervals advantages are available in feasibility of using fuel of lower enrichment and in frequent replacement of a relatively small portion of the fuel assemblies during each refueling. In addition the concentration of neutron absorbing material in the medium in which the core is immersed, for example boron, in water, may be lower.
Typically in a pressurized water reactor annual refueling, demanding enrichment in fissionable material of the order of 3.2% requires a concentration in the water of about 1200 parts per million of natural boron typically including by weight 20% B.sup.10 and 80% B.sup.11 at the beginning of life with a consequent build-up of a high concentration of tritium, H.sup.3, during life while the concentration of boron is being reduced to about 10 p.p.m. In a typical example, for a semiannual refueling cycle the initial enrichment is reduced and the concentration of boron at the start of life is reduced to 650 p.p.m. with consequent reduction in the H.sup.3 generated; and for a three-months refueling cycle the enrichment is only 2.7% and the boron concentration at the start of life is reduced to 350 p.p.m.
It has been found that by refueling during an interval of three days about every three months a fuel saving amounting to about $14 per kilowatt can be realized.
It is an object of this invention to improve the economy of operation of a nuclear reactor and to provide a method of refueling a nuclear reactor which can be carried out in a short interval of only a few days permitting a short time cycle, of the order of three or six months between refuelings of the reactor and consequent reduction of initial enrichment in fissionable material of the fuel and low concentration of neutron absorbers, and with minimized reactor downtime for refueling.